Devices for plastic injection molding employ various mold open-close devices. For example, a double-plate mold closing mechanism is employed in the mold closing mechanism of a double-plate injection molding machine, as disclosed in Chinese patent CN2186650. The mold closing mechanism comprises a moveable platen, a fixed platen and four tie bars arranged between the moveable platen and the fixed platen. Each of the tie bars is provided with a pair of flashboards at the side adjacent to the fixed platen. Two sets of clamping devices are provided with one located between two of the pairs of flashboards and the other located between the other two pairs of flashboards. The clamping devices are provided horizontally, and the two pairs of flashboards connected with the clamping devices are also provided horizontally. Each pair of flashboards is composed of a flashboard on the left and a flashboard on the right. The flashboard on the left comprises a left end surface vertical to the horizontal plane and a right arc surface matching the surface of the tie bar, and the flashboard on the right comprises a right end surface vertical to the horizontal plane and a left arc surface matching the surface of the tie bar. The two left flashboards of the two pairs are connected by a stud, and the two right flashboards are connected by another stud. The clamping device comprises a hydraulic cylinder, which is fixed directly to the right end surface of the right flashboard of the left pair, while the piston rod of the hydraulic cylinder is fixed directly to the left end surface of the left flashboard of the right pair. The hydraulic cylinder is controlled to drive the piston rod to extend or contract so that the left and right flashboards of each pair may be driven to move towards the surface of the tie bar simultaneously to close the flashboards, or to move against the surface of the tie bar simultaneously to open the flashboards. And the heads of the four tie bars are respectively provided with a notch corresponding to the flashboards for securing the position of the tie bars and the flashboards. Also, a double-plate combined with direct-driving mold clamping structure is disclosed by Chinese patent CN2737532. The structure comprises a fixed platen, a moveable platen and four tie bars connected with the fixed platen and the moveable platen. Cogs are configured to be around the periphery of an end portion of each tie bar. A clamping unit is provided on each pillar adjacent to the moveable platen. The clamping unit is constructed by arranging two clamping blocks inside a clamping base. The opposite faces of the two clamping blocks are configured to have shrinking openings corresponding to the periphery of tie bar. The inner sides of the shrinking openings are provided with teeth engaging the cogs. A hydraulic cylinder is provided outside the clamping blocks, and a guide bar is provided inside the two opposite clamping blocks so that two clamping blocks are driven to open or close when the hydraulic cylinder operates.
It can be seen that the movable platen and the tie bars' axis (i.e., the moving direction of the mold) of the existing injection molding machine are orientated by lock nuts which are directly driven by a hydraulic cylinder. The clamping force of the nuts is directly provided by the hydraulic cylinder. Thus, the claiming force depends directly on the size of the hydraulic cylinder. With this structure, a large size of the hydraulic cylinder is required in order to provide large driving force. That not only increases the production cost but also causes a waste of energy.